1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser, and more particularly, to a vertical cavity surface emitting laser capable of operating predominantly in a single fundamental transverse mode.
2. Description of the Related Art
Vertical cavity surface emitting lasers (VCSELs) provide a higher coupling efficiency with an optical fiber due to the emission of a circular laser beam, and the ease of the manufacture of an array, and enable error detection and characteristics measurement in a wafer state, so they are emerging as a promising light source in optical communications and optical interconnections. In particular, index-guided VCSELs, in which an aperture through which current flows is confined by selective oxidation, have a very low threshold current and power consumption, high efficiency, and excellent linearity of current to light output. Therefore, the index-guided VCSELs can be applied to transceiver modules for local area communications and for optical interconnection between computers and digital displays, and some of which are under commercialization.
However, index-guided VCSELs have a greater difference in the index of refractivity between an oxidized portion and a non-oxidized portion so that they principally can operate in a plurality of traverse modes. Since the oxidized portion of the index-guided VCSEL provides a structure of current injection confinement, it is easy to inject the current into a circumference portion of an aperture surrounded by the oxidized region and thus higher order traverse modes tend to be oscillated at low output power. Undoubtedly, the operation efficiency of a single transverse mode can be increased to some extent by forming the aperture surrounded by the oxidized region to be smaller. In this case, resistance of the VCSEL excessively increases, and a cross-sectional area for the laser oscillation becomes small, thereby lowering the overall output power. As a result, it is difficult to apply the index-guided VCSEL to optical communications devices. For these reasons, a variety of approaches to suppress multiple transverse modes and improve the output power of the single transverse mode oscillation in the index-guided VCSEL have been suggested. Suggestions for the improvement of the single transverse mode oscillation output power include a method of etching an external portion of a laser beam output window using ion beams, a method of implanting ions while selective oxidation is performed, a method of forming a resonator with an active layer to be longer, etc. However, those methods suggested up to now have problems of complicated processes and poor reproducibility.
To solve the above-described problems, it is an object of the present invention to provide a vertical cavity surface emitting laser (VCSEL) capable of operating predominantly in a single fundamental transverse mode, which can be manufactured by a simple process and has excellent reproducibility.
To achieve the object of the present invention, there is provided a VCSEL comprising a feedback member which feeds a portion of light externally emitted from a resonator for generating and amplifying a laser beam, back into the resonator by reflecting the same.
Preferably, the feedback member acts as a concave mirror for the resonator. Preferably, the vertical cavity surface emitting laser is constructed of a material based on one selected from the group consisting of GaAs, GaSb, GaN, InP, and ZnSe. Preferably, the feedback member is formed of a semiconductor material which is transparent with respect to the wavelength of the laser oscillation and is capable of lattice matching with a material of the substrate or a material of a thin film structure on the substrate. Preferably, the feedback member is formed of a material selected from the group consisting of organic and inorganic materials which are transparent with respect to the wavelength of the laser oscillation.
It is preferable that the feedback member is positioned on the top of an upper distributed Bragg reflector and/or the bottom of the substrate. In this case, the feedback member positioned on the top of the upper distributed Bragg reflector has an effective radius of curvature ranging from approximately 200 xcexcm to approximately 500 xcexcm. Preferably, a gain area for the laser oscillation is increased for high-power oscillation of a fundamental transverse mode by increasing at least one of an aperture of laser oscillation, the effective diameter of the feedback member, and a radius of curvature of the feedback member. Preferably, at least one of the aperture of laser oscillation and the effective diameter of the feedback member is increased to weaken the diffraction power of oscillating light such that the feedback member feeds the portion of the light externally emitted from the resonator back into the resonator by reflecting the same and simultaneously acts as a lens for light proceeding to the outside from the resonator to make it into a converging beam, diverging beam, or parallel beam. Preferably, the laser oscillation is induced by current injection. In this case, the VCSEL further may comprise a current confinement structure which is formed by selective oxidation or ion implantation and confines flow of current. Alternatively, the laser oscillation may be induced by optical pumping of external light.